When a voltage with the same polarity is continuously applied to pixels, liquid crystal display devices might suffer some failure, such as burn-in, and therefore they employ drive methods in which the polarity of the voltage applied to the pixels is changed every predetermined period. Examples of the methods used include frame inversion drive in which the voltage polarity is changed every frame, line inversion drive in which the voltage polarity is changed every line or every several lines, and dot inversion drive in which the voltage polarity is changed for each pixel. Also, in order to improve response speed, some liquid crystal display devices perform overshoot drive (also referred to as “overdrive” or “overdriving”), applying a voltage higher or lower than the voltage that should be applied to pixels based on a video signal for the current frame and a video signal for the previous frame.
FIG. 13 is a block diagram illustrating the configuration of a conventional liquid crystal display device that performs line inversion drive and overshoot drive. In FIG. 13, a correcting circuit 90 includes a frame memory 91, a look-up table 92, and a correction process portion 93. The frame memory 91 stores a video signal of one frame, and the look-up table 92 has stored therein correction values emphasizing temporal signal change. Based on a video signal X for the current frame supplied from a signal source S and a video signal Y for the previous frame stored in the frame memory 91, the correction process portion 93 reads a correction value from the look-up table 92, and outputs the correction value being read as a correction video signal V.
A display control circuit 1, a scanning signal line drive circuit 2, and a data signal line drive circuit 3 drive scanning signal lines G1 to Gn and data signal lines S1 to Sm based on the correction video signal V, and a control signal C1 supplied from the signal source S by way of the correcting circuit 90, thereby performing line inversion drive on a pixel array 5 including pixels 6. A common electrode drive circuit 4 applies a common electrode voltage Vcom to a common electrode 7 provided in the pixel array 5.
Referring to FIGS. 14A to 14D, effects of overshoot drive will be described. FIGS. 14A to 14D show changes of the voltage applied to the pixels and changes in pixel intensity in the case where the intensity is increased from an initial value Li to a target value Lt within one frame period from time t1 to time t2. When overshoot drive is not performed, a voltage Vt corresponding to the target value Lt for the intensity is applied to the pixels during the period from time t1 to time t2 (FIG. 14A). Accordingly, the intensity approximates the target value Lt at a certain speed (FIG. 14B). However, depending on the combination of the initial value Li and the target value Lt for the intensity, the intensity might not reach the target value Lt within one frame period. At time t2 in the example shown in FIG. 14B, the intensity only reaches a level that is lower than the target value Lt by ΔL.
On the other hand, when overshoot drive is performed, a voltage Vo higher than the voltage Vt is applied to the pixels during the period from time t1 to time t2 (FIG. 14C). Accordingly, the intensity approximates the target value Lt at a higher speed than when overshoot drive is not performed (FIG. 14D). Therefore, by applying a voltage at a suitable level in accordance with the combination of the initial value Li and the target value Lt for the intensity, it becomes possible to allow the intensity to reach the target value Lt within one frame period. At time t2 in the example shown in FIG. 14D, the intensity coincides with the target value Lt. Note that when the target value for the intensity is lower than the initial value, a voltage lower than the voltage corresponding to the target value for the intensity is applied to the pixels.
Overshoot drive is disclosed in, for example, Patent Document 1. In addition, Patent Document 2 discloses technology for passive-matrix liquid crystal display devices having their response speeds changed according to the polarity of an applied voltage, in which two types of signals are used to generate a pixel signal for maximizing a torque applied to liquid crystal molecules during switching.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2001-265298
[Patent Document 2] Japanese Laid-Open Patent Publication No. 10-54972